During cardiovascular surgical procedures, the heart is often arrested and the patient is placed on cardiopulmonary bypass. In addition, a subset of patients with cardiopulmonary complications and or disease will be placed on partial longer-term cardiopulmonary bypass. These patients include, but are not limited to: neonates with severe pulmonary lung disease, bridge to transplant patients, liver transplant patients and patients with severe myocardial trauma accompanied by pump failure. Such cardiopulmonary bypass is used to support the patient's circulation and/or pulmonary function while the heart is being surgically repaired or the failing organ is allowed to recover. Typical surgical repair procedures include valve replacement, annuloplasty, coronary artery bypass grafting, total heart replacement, cardiac assist placement, repair of tetralogy of Fallot, repair of atrial and ventricular septal defects, heart and/or lung transplantation, liver transplantation and the like. Cardiopulmonary bypass devices use a cannula to remove blood from the patient where it is oxygenated, purged of carbon dioxide, heated or cooled, filtered and pumped back into the systemic circulation of the patient. Blood filters are used in the cardiopulmonary bypass system to trap particulates and gas bubbles that are generated in the extracorporeal loop. Blood filters prevent particulates and gas bubbles from being pumped back into the patient. The most common gas entrained within the blood of an extracorporeal circuit is air. Such particulates and gas bubbles, also known as emboli, can cause blockage in the arterioles and capillary beds and lead to ischemic cell death. Consequences of such ischemic cell death may affect organ function (viz. intestine, pancreas, kidney, brain, etc.) and result in sepsis, renal failure and neurological defects such as loss of memory, cognitive function, and changes in personality.
Modern blood filters do trap emboli and remove debris before they are pumped back into patients but it has been scientifically validated that small gas bubbles, primarily air, and certain particulate substances missed by these filters are returned to the patients and compromise patient recovery. Patients who undergo cardiopulmonary bypass are often subject to some degree of neurological deficit as a result of the gas bubbles and other embolic materials. This phenomenon is sometimes characterized as “Pump Head”.
Current blood filters are considered to be adequate for removing larger debris and large gas bubbles from the blood, but patient outcomes would be improved if small gas bubble and particulate removal efficiencies were higher. One of the primary problems with current blood filters is that when the mesh size is increased to screen out smaller particles and bubbles, the pressure drop across the filter becomes unacceptably high at normal blood flow rates. Such unacceptably high pressure gradients can potentially cause tubing or connection failures resulting in blood leaks or air leaks into the system, either of which could be catastrophic. Typical examples of the prior art in blood filters include U.S. Pat. No. 4,919,802 to Katsura, U.S. Pat. No. 4,411,783 to Dickens et al., U.S. Pat. No. 5,279,550 to Habib et al., U.S. Pat. No. 5,632,894 to White et al., and U.S. Pat. No. 5,683,355 to Fini et al. These patents disclose filters and bubble traps that are static devices employing filter screens to collect the debris and bubbles.
Additionally, U.S. Pat. Nos. 4,411,783, 4,919,802, and 5,632,894 disclose use of tangential blood inflow and a gas vent at the top center of the filter to improve bubble removal. The tangential inflow generates centrifugal effects to move the bubbles to the center of the device. However, since these are not active systems, they are unable to generate the rotational velocities necessary to adequately rid the blood of small bubbles that can cause neurological defects. A recent publication by Schoenburg (Ref J Thorac Cardiovasc Surg 2003:126:1455-60) describes an air bubble trap, which incorporates a three channel helix to cause the blood to passively rotate around the axis of the tube causing the centrifugal forces to direct air bubbles to the center of the flow stream where they are evacuated via a special collection tube. None of these devices impart rotational motion using an active drive system, which can rotate the blood at much higher rates and thus generate higher separation forces on the bubbles to remove them from the blood.
New devices and methods are needed to more efficiently remove gas bubbles from the blood of a patient undergoing circulatory support without traumatizing blood elements and without unacceptably increasing the pressure drop across the filter to dangerous levels.